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It was believed for a long time that the human brain didn’t change much once mature. That belief has been turned on its head over the course of the past fifteen years. Brain & Behavior Research Foundation NARSAD Grantee Bruce S. McEwen, Ph.D. demonstrated in 1998 that the brain can “remodel” its architecture, adapts to experience and is very “plastic.” Brain & Behavior Research Foundation Scientific Council Member Fred H. Gage, Ph.D. and colleagues also demonstrated in 1998 that there is “neurogenesis” (the birth of new neurons) in the human hippocampus―a brain area that is key to memory and learning and one that can play a role in the development of depression. With this new study, for the first time, there is a quantification of the number of new neurons produced in adult humans.

NARSAD Grantee Kirsty Spalding, Ph.D., and team at the Karolinska Institutet in Stockholm, Sweden, used an innovative methodology to identify the “birth date” of neurons in deceased human brains. They found a way to “carbon date” neurons by testing brain specimens from deceased adult humans who lived through the middle of the last century, during a period of above-ground nuclear bomb testing and elevated atmospheric levels of carbon-14. The researchers were able to quantify the amount of carbon-14 stamped into DNA when a new neuron is born, essentially “carbon dating” the neurons. (Dr. Spalding and team have used the methodology over the past decade to test a variety of cells, including fat cells, and were able to refine it to a point that it became sensitive enough to measure tiny amounts of carbon 14 in small hippocampus samples.) They found that more than one-third of neurons are regularly renewed throughout life―about 1,400 are added each day during adulthood, and this rate declines only modestly with age (hippocampus neurons also die each day, so the overall number remains more or less in balance). Their findings were published June 6, 2013 in Cell.

This research supports the idea that new neurons may support cognitive functions throughout life and that treatments to enhance neurogenesis may help treat psychiatric illnesses such as depression that are believed to be associated with reduced hippocampal neurogenesis.

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